1. The Field of the Invention
The present invention generally relates to x-ray tube devices. In particular, the present invention relates to coatings, and coating procedures, that can be used in the manufacture of x-ray tube components.
2. The Related Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis.
Regardless of the applications in which they are employed, x-ray devices operate in similar fashion. In general, x-rays are produced when electrons are emitted, accelerated, and then impinged upon a material of a particular composition. This process typically takes place within an evacuated enclosure of an x-ray tube.
The evacuated enclosure portion of an x-ray tube can be implemented in any one of a number of ways. For example, one common implementation includes one portion that is formed of a heat-conductive material, such as copper. A second portion comprises a glass or ceramic material. The two portions are then hermetically sealed together so as to maintain a vacuum within the resulting enclosure (sometimes referred to as the “can”).
Disposed within the evacuated enclosure is a cathode, or electron source, and an anode oriented to receive electrons emitted by the cathode. The anode can be stationary within the tube, or can be in the form of a rotating annular disk that is mounted to a rotor shaft which, in turn, is rotatably supported by ball bearings contained in a bearing assembly.
In operation, an electric current is supplied to a filament portion of the cathode, which causes a stream of electrons to be emitted via a process known as thermionic emission. A high voltage potential is placed between the cathode and anode to cause the electrons to form a stream and accelerate towards a target surface located on the anode. Upon striking the target surface, some of the resulting kinetic energy is released in the form of electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials with high atomic numbers (“Z numbers”) are typically employed. The x-rays are then collimated so that they exit the x-ray tube through a window in the tube, and enter the x-ray subject, such as a medical patient.
As discussed above, some of the kinetic energy resulting from the collision with the target surface results in the production of x-rays. However, much of the kinetic energy is released in the form of heat. Still other electrons simply rebound from the target surface and strike other “non-target” surfaces within the x-ray tube. These are often referred to as “backscatter” electrons. These backscatter electrons retain a significant amount of kinetic energy after rebounding, and when they also impact other non-target surfaces they generate large amounts of heat.
Heat generated from these target and non-target electron interactions can reach extremely high temperatures and must be reliably and continuously removed. If left unchecked, it can ultimately damage the x-ray tube and shorten its operational life. Some x-ray tube components, like ball bearings housed in the bearing assembly, are especially sensitive to heat and are easily damaged. For instance, high temperatures can melt the thin metal lubricant that is typically present on the ball bearings, exposing them to excessive friction. Additionally, repeated exposure to these high temperatures can degrade the bearings, thereby reducing their useful life as well as that of the x-ray tube.
These problems related to high temperatures produced in the x-ray tube have been addressed in a variety of ways. For example, rotating anodes are used to effectively distribute heat. The circular face of a rotating anode that is directly opposed to the cathode is called the anode target surface. The focal track comprising a high-Z material is formed on the target surface. During operation, the anode and rotor shaft supporting the anode are spun at high speeds, thereby causing successive portions of the focal track to continuously rotate in and out of the electron beam emitted by the cathode. The heating caused by the impinging electrons is thus spread out over a larger area of the target surface and the underlying anode.
While the use of the rotating anode is effective in reducing the amount of heat present on the anode, high levels of heat are still typically present. Thus, cooling structures are often employed to further absorb and dissipate additional heat from the anode. Once absorbed, the heat is typically conveyed to the evacuated enclosure surface, where it is then absorbed by a circulated coolant. One example of such an arrangement utilizes cooling fins that are placed adjacent to the anode. During tube operation heat is transferred from the anode to the evacuated enclosure surface via the cooling fins and then absorbed by the circulating coolant.
Another attempt to dissipate heat in x-ray tubes involves the use of more massive anode structures, enabling a given amount of conducted heat to be spread throughout a larger volume than that available in smaller anodes. Unfortunately, larger anodes require correspondingly more massive rotor assemblies to support the increased mass and rotational inertia of the anode. This in turn creates a larger conductive heat path from the anode, through the rotor shaft, and into the bearings in the rotor assembly, thus causing unwanted bearing heating.
The above cooling practices, while effective for general heat removal, can be insufficient by themselves to prevent heat from passing from the anode, through the rotor shaft, and into the bearings and other areas of the tube—especially in today's higher power x-ray tubes. As discussed before, this heat is highly detrimental to the bearings, and to other components within the x-ray tube.
Another method to reduce the effects is of high operating temperatures is to provide x-ray tube components with coatings that exhibit improved thermal characteristics. For instance, emissive coatings have been applied to various anode surfaces to enhance the rate of heat transferred from the anode. Additionally, an absorptive coating may be disposed, for example, on the inside surface of the evacuated enclosure to enhance the absorption by the enclosure of heat emitted by the anode, and the subsequent transfer of that heat to the can exterior where it may be removed by the circulated coolant. This absorptive coating has typically comprised a thin layer of iron that is mechanically bonded to the inner surface of the evacuated enclosure or housing.
The use of such coatings has not been completely successful however. For instance, over time the repeated cycles of heating and cooling may cause absorptive coatings to flake or spall away from the coated surface. This debris can then contaminate other components within the x-ray tube, and lead to its premature failure. Moreover, there is often a thermal mismatch between the surface of the coated component and the absorptive coating, which tends to weaken the bond between the two materials as they thermally expand during use. Again, this leads to undesired flaking and spalling and the consequent contamination of the x-ray tube.
The flaking and spalling described above may also cause electrical arcing within the evacuated x-ray tube, which may result in severe electrical damage to a number of x-ray tube components and/or failure of the x-ray device.
Many of the above problems associated with flaking and spalling are exacerbated when the coating is mechanically bonded to the x-ray tube component. For example, an absorptive coating comprising an iron plating may be mechanically bonded to a surface of an evacuated can comprising copper by immersing the can in a bath comprising iron solution. Such a mechanical bond existing between an absorptive coating and the inner surface of the evacuated can is a relatively low-strength bond. The relative weakness of the mechanical bond may cause the absorptive layer to flake away when the can is subjected to relatively small amounts of thermal or mechanical stress.
The above situation is made worse when mechanically bonded coatings are employed in high-power x-ray tubes. These high-power x-ray tubes are capable of higher operating temperatures and longer operation times than standard x-ray tubes. This, in turn, results in increased mechanical and thermal stress on tube components, including the rotating anode and the evacuated can enclosure. This increased stress serves only to increase the incidence of flaking or spalling of the absorptive coatings, especially those that are applied to the inner surface of the evacuated can.
Another drawback encountered with coatings that are mechanically bonded to the evacuated can relates to the preparation work required to apply the coating. For example, before mechanically bonding an absorptive coating to the inner surface of the evacuated can, grit blasting of the can surface is often necessary in order to prepare the surface for adhesion of the coating. In grit blasting, the surface to be treated is blasted with high velocity, irregularly sized bits of metal, such as aluminum dioxide or other suitable material, in order to give it a roughened surface that enhances the adhesion of the iron to the can surface. While effective at preparing the can surface, grit blasting may also temporarily embed grits into the can surface. Later, during operation of the x-ray tube, grit particles may work free from the inner can surface and contaminate the volume of the evacuated tube. These particles pose a contamination and/or electrical arcing risk similar to the risk posed by the flaking of the absorptive coating, as described above.
Another drawback related to the mechanical bonding of the absorptive coating to the evacuated can relates to the fact that less control is achieved as to where the absorptive coating is applied to the inner surface of the can. Thus, a technician is prevented from precisely controlling application of the absorptive coating, which results in increased cost and waste during tube manufacture.
What is needed, therefore, is an x-ray tube that withstands the destructive heat produced within it during use, thus protecting its components. Also desired is a method which more efficiently dissipates heat produced by the anode to the evacuated enclosure and away from heat sensitive tube components. Further, a method for applying absorptive or other coatings to the surface of the evacuated enclosure, such that flaking or spalling is reduced or eliminated, is also needed. Also, any solution to the above should enable greater control to be exercised as to where the absorptive coating is applied to the evacuated enclosure.